Method and apparatus for video coding

ABSTRACT

Aspects of the disclosure provide methods and an apparatus for video decoding. In some embodiments, the apparatus for video decoding includes processing circuitry. The processing circuitry decodes at least one syntax element from a coded video bitstream. The at least one syntax element is indicative of a block size of a non-square block under reconstruction that has a rectangular shape. The block size includes a first size in a first dimension and a second size in a second dimension. The first size is different from the second size. The processing circuitry predicts a sample of the non-square block based on a first set of intra prediction directions for the non-square block. The first set of intra prediction directions does not include at least one of a second set of intra prediction directions for a square block.

INCORPORATION BY REFERENCE

This present disclosure claims the benefit of priority to U.S.Provisional Application No. 62/679,664, filed on Jun. 1, 2018, which isincorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GByte of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reducing aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signal is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision contribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

A video encoder and decoder can utilize techniques from several broadcategories, including, for example, motion compensation, transform,quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In Intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, the picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be an intra picture. Intra pictures and their derivationssuch as independent decoder refresh pictures, can be used to reset thedecoder state and can, therefore, be used in as the first picture in acoded video bitstream and a video session, or as a still image. Thesamples of an intra block can be exposed to a transform, and thetransform coefficients can be quantized before entropy coding. Intraprediction can be a technique that minimizes sample values in thepre-transform domain. In some cases, the smaller the DC value after atransform is, and the smaller the AC coefficients are, the fewer bitsare required at a given quantization step size to represent the blockafter entropy coding.

Traditional intra coding such as known from, for example MPEG-2generation coding technologies does not use intra prediction. However,some newer video compression technologies include techniques thatattempt, from, for example, surrounding sample data and/or metadataobtained during the encoding/decoding of spatially neighboring, andpreceding in decoding order, blocks of data. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction is only using reference data from thecurrent picture under reconstruction and not from reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques can be used in a given video coding technology,the technique in use can be coded in an intra prediction mode. Incertain cases, modes can have submodes and/or parameters, and those canbe coded individually or included in the mode codeword. Which codewordto use for a given mode/submode/parameter combination can have an impactin the coding efficiency gain through intra prediction, and so can theentropy coding technology used to translate the codewords into abitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), benchmark set(BMS). A predictor block can be formed using neighboring samples valuesbelonging to already available samples. Sample values of neighboringsamples are copied into the predictor block according to a direction. Areference to the direction in use can be coded in the bitstream or mayitself be predicted.

Referring to FIG. 1, depicted in the lower right is a subset of ninepredictor directions known from H.265's 33 possible predictordirections. The point where the arrows converge (101) represents thesample being predicted. The arrows represent the direction from whichthe sample is being predicted. For example, arrow (102) indicates thatsample (101) is predicted from a sample or samples to the upper right,at a 45 degree angle from the horizontal. Similarly, arrow (103)indicates that sample (101) is predicted from a sample or samples to thelower right of sample (101), in a 22.5 degree angle from the horizontal.

Still referring to FIG. 1, on the top left there is depicted a squareblock (104) of 4×4 samples (indicated by a dashed, boldface line). Thesquare block (104) includes 16 samples, each labelled with an “S”, itsposition in Y dimension (e.g., row index) and its position in Xdimension (e.g., column index). For example, a sample S21 is the secondsample in Y dimensions (from the top) and the first (from the left)sample in X dimension. Similarly, a sample S44 is the fourth sample inthe block (104) in both Y and X dimension. As the block (104) is 4×4samples in size, the sample S44 is at the bottom right. Further shownare reference samples, that follow a similar numbering scheme. Areference sample is labelled with an R, its Y position (e.g., row index)and X position (column index) relative to the block (104). In both H.264and H.265, prediction samples neighbor the block under reconstruction;therefore no negative values need to be used.

Intra picture prediction can work by copying reference sample valuesfrom the neighboring samples as appropriated by the signaled predictiondirection. For example, assume the coded video bitstream includessignaling that, for this block, indicates a prediction directionconsistent with arrow (102)—that is, samples are predicted fromprediction sample or samples to the upper right, at a 45 degree anglefrom the horizontal. In that case, samples S41, S32, S23, and S14 arepredicted from same R05. Sample S44 is then predicted from R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation, in order to calculate areference sample; especially when the directions are not evenlydivisible by 45 degrees.

The number of possible directions has increased as video codingtechnology developed. In H.264 (year 2003), nine different directioncould be represented. That increased to 33 in H.265 (year 2013), andJEM/VVC/BMS, at the time of disclosure, can support up to 65 directions.Experiments have been conducted to identify the most likely directions,and certain techniques in the entropy coding are used to represent thoselikely directions in a small number of bits, accepting a certain penaltyfor less likely directions. Further, the directions themselves can besometimes be predicted from neighboring directions used in neighboring,already decoded, blocks.

FIG. 2 is a schematic 201 that depicts 67 prediction modes according toJEM to illustrate the increasing number of prediction directions overtime.

The mapping of an intra prediction directions bits in the coded videobitstream that represent the direction can be different form videocoding technology to video coding technology; and can range, forexample, from simple direct mappings of prediction direction to intraprediction mode to codewords, to complex adaptive schemes involving mostprobably modes and similar techniques. A person skilled in the art isreadily familiar with those techniques. In all cases, however, there canbe certain directions that are statistically less likely to occur invideo content than certain other directions. As the goal of videocompression is the reduction of redundancy, those less likely directionswill, in a well working video coding technology, be represented by alarger number of bits than more likely directions.

SUMMARY

Aspects of the disclosure provide methods and an apparatus for videodecoding. In some embodiments, the apparatus for video decoding includesprocessing circuitry. The processing circuitry decodes at least onesyntax element from a coded video bitstream. The at least one syntaxelement is indicative of a block size of a non-square block underreconstruction that has a rectangular shape. The block size includes afirst size in a first dimension and a second size in a second dimension.The first size is different from the second size. The processingcircuitry predicts a sample of the non-square block based on a first setof intra prediction directions for the non-square block. The first setof intra prediction directions does not include at least one of a secondset of intra prediction directions for a square block.

In some embodiments, the at least one of the second set of intraprediction directions covers a subrange of an angular range covered bythe second set of intra prediction directions. In an example, thesubrange includes a first end of the angular range.

In some embodiments, intra prediction modes used for the first set ofintra prediction directions do not include at least one intra predictionmode used for the at least one of the second set of intra predictiondirections.

In some embodiments, at least one intra prediction mode used for the atleast one of the second set of intra prediction directions is assignedto at least one of the first set of intra prediction directions. The atleast one of the first set of intra prediction directions is notincluded in the second set of intra prediction directions.

In some embodiments, in a combined angular range covered by the firstset of intra prediction directions and the at least one of the secondset of intra prediction directions, the at least one of the first set ofintra prediction directions is covered by a subrange that includes afirst end of the combined angular range. The at least one of the secondset of intra prediction directions is covered by another subrange thatincludes a second end of the combined angular range.

In some embodiments, a first angular range is covered by the first setof intra prediction directions, and a second angular range is covered bythe second set of intra prediction directions. A subrange of the firstangular range covered by the at least one of the first set of intraprediction directions is outside the second angular range.

In some embodiments, a number of the at least one of the first set ofintra prediction directions is equal to a number of the at least one ofthe second set of intra prediction directions.

In some embodiments, a number of intra prediction modes used for thesecond set of intra prediction directions for the square block is equalto a number of intra prediction modes used for the first set of intraprediction directions for the non-square block.

In some embodiments, the at least one of the second set of intraprediction directions is based on a shape of the non-square block.

In some embodiments, the at least one of the second set of intraprediction directions is based on an aspect ratio of the non-squareblock.

Aspects of the disclosure also provide a non-transitorycomputer-readable storage medium storing a program executable by atleast one processor for video decoding to perform any of the methods forvideo decoding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a schematic illustration of exemplary intra prediction modes.

FIG. 2 is another illustration of exemplary intra prediction modes.

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system (300) in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of acommunication system (400) in accordance with an embodiment.

FIG. 5 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 6 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 7 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 8 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 9 shows a schematic illustration of an intra prediction of samplesin a non-square block in accordance with an embodiment.

FIG. 10 shows a schematic illustration of an intra prediction of samplesin a non-square block in accordance with an embodiment.

FIG. 11 shows a schematic illustration of an intra prediction of samplesin a non-square block in accordance with an embodiment.

FIG. 12 shows a flow chart outlining a process (1200) according to someembodiments of the disclosure.

FIG. 13 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the present disclosure are directed to improving theintra prediction of non-square blocks by, for example, referring toreference samples spatially close to the block under reconstruction.Further, in some embodiments, this process is performed by minimizingthe number of directions used.

FIG. 3 illustrates a simplified block diagram of a communication system(300) according to an embodiment of the present disclosure. Thecommunication system (300) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (350). Forexample, the communication system (300) includes a first pair ofterminal devices (310) and (320) interconnected via the network (350).In the FIG. 3 example, the first pair of terminal devices (310) and(320) performs unidirectional transmission of data. For example, theterminal device (310) may code video data (e.g., a stream of videopictures that are captured by the terminal device (310)) fortransmission to the other terminal device (320) via the network (350).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (320) may receive the codedvideo data from the network (350), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (300) includes a secondpair of terminal devices (330) and (340) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (330) and (340)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (330) and (340) via the network (350). Eachterminal device of the terminal devices (330) and (340) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (330) and (340), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 3 example, the terminal devices (310), (320), (330) and(340) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (350) represents any number ofnetworks that convey coded video data among the terminal devices (310),(320), (330) and (340), including for example wireline (wired) and/orwireless communication networks. The communication network (350) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(350) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 4 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (413), that caninclude a video source (401), for example a digital camera, creating forexample a stream of video pictures (402) that are uncompressed. In anexample, the stream of video pictures (402) includes samples that aretaken by the digital camera. The stream of video pictures (402),depicted as a bold line to emphasize a high data volume when compared toencoded video data (404) (or coded video bitstreams), can be processedby an electronic device (420) that includes a video encoder (403)coupled to the video source (401). The video encoder (403) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (404) (or encoded video bitstream (404)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (402), can be stored on a streamingserver (405) for future use. One or more streaming client subsystems,such as client subsystems (406) and (408) in FIG. 4 can access thestreaming server (405) to retrieve copies (407) and (409) of the encodedvideo data (404). A client subsystem (406) can include a video decoder(410), for example, in an electronic device (430). The video decoder(410) decodes the incoming copy (407) of the encoded video data andcreates an outgoing stream of video pictures (411) that can be renderedon a display (412) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (404),(407), and (409) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Codingor VVC. The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (420) and (430) can includeother components (not shown). For example, the electronic device (420)can include a video decoder (not shown) and the electronic device (430)can include a video encoder (not shown) as well.

FIG. 5 shows a block diagram of a video decoder (510) according to anembodiment of the present disclosure. The video decoder (510) can beincluded in an electronic device (530). The electronic device (530) caninclude a receiver (531) (e.g., receiving circuitry). The video decoder(510) can be used in the place of the video decoder (410) in the FIG. 4example.

The receiver (531) may receive one or more coded video sequences to bedecoded by the video decoder (510); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (501), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (531) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (531) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (515) may be coupled inbetween the receiver (531) and an entropy decoder/parser (520) (“parser(520)” henceforth). In certain applications, the buffer memory (515) ispart of the video decoder (510). In others, it can be outside of thevideo decoder (510) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (510), forexample to combat network jitter, and in addition another buffer memory(515) inside the video decoder (510), for example to handle playouttiming. When the receiver (531) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (515) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (515) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (510).

The video decoder (510) may include the parser (520) to reconstructsymbols (521) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (510),and potentially information to control a rendering device such as arender device (512) (e.g., a display screen) that is not an integralpart of the electronic device (530) but can be coupled to the electronicdevice (530), as was shown in FIG. 5. The control information for therendering device(s) may be in the form of Supplementary EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (520) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (520) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (520) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (520) may perform entropy decoding/parsing operation on thevideo sequence received from the buffer memory (515), so as to createsymbols (521).

Reconstruction of the symbols (521) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (520). The flow of such subgroup control information between theparser (520) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (510)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (551). Thescaler/inverse transform unit (551) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (521) from the parser (520). The scaler/inversetransform unit (551) can output blocks comprising sample values, thatcan be input into aggregator (555).

In some cases, the output samples of the scaler/inverse transform (551)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (552). In some cases, the intra pictureprediction unit (552) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (558). The currentpicture buffer (558) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(555), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (552) has generated to the outputsample information as provided by the scaler/inverse transform unit(551).

In other cases, the output samples of the scaler/inverse transform unit(551) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (553) canaccess reference picture memory (557) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (521) pertaining to the block, these samples can beadded by the aggregator (555) to the output of the scaler/inversetransform unit (551) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (557) from where themotion compensation prediction unit (553) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (553) in the form of symbols (521) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (557) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (555) can be subject to variousloop filtering techniques in the loop filter unit (556). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (556) as symbols (521) from the parser (520), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (556) can be a sample stream that canbe output to the render device (512) as well as stored in the referencepicture memory (557) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (520)), the current picture buffer (558) can becomea part of the reference picture memory (557), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (510) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as document in thevideo compression technology or standard. Specifically, a profile canselect a certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example mega samples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (531) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (510) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 6 shows a block diagram of a video encoder (603) according to anembodiment of the present disclosure. The video encoder (603) isincluded in an electronic device (620). The electronic device (620)includes a transmitter (640) (e.g., transmitting circuitry). The videoencoder (603) can be used in the place of the video encoder (403) in theFIG. 4 example.

The video encoder (603) may receive video samples from a video source(601) (that is not part of the electronic device (620) in the FIG. 6example) that may capture video image(s) to be coded by the videoencoder (603). In another example, the video source (601) is a part ofthe electronic device (620).

The video source (601) may provide the source video sequence to be codedby the video encoder (603) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any color space (for example, BT.601 Y CrCB, RGB, . . . )and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (601) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (601) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (603) may code andcompress the pictures of the source video sequence into a coded videosequence (643) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (650). In some embodiments, the controller(650) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (650) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (650) can be configured to have other suitablefunctions that pertain to the video encoder (603) optimized for acertain system design.

In some embodiments, the video encoder (603) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (630) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (633)embedded in the video encoder (603). The decoder (633) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (634). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (634) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (633) can be the same as of a“remote” decoder, such as the video decoder (510), which has alreadybeen described in detail above in conjunction with FIG. 5. Brieflyreferring also to FIG. 5, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (645) and the parser (520) can be lossless, the entropy decodingparts of the video decoder (510), including the buffer memory (515), andparser (520) may not be fully implemented in the local decoder (633).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (630) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (632) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (633) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (630). Operations of the coding engine (632) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 6), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (633) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (634). In this manner, the video encoder(603) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (635) may perform prediction searches for the codingengine (632). That is, for a new picture to be coded, the predictor(635) may search the reference picture memory (634) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(635) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (635), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (634).

The controller (650) may manage coding operations of the source coder(630), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (645). The entropy coder (645)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies known to a person skilled in the art as, for exampleHuffman coding, variable length coding, arithmetic coding, and so forth.

The transmitter (640) may buffer the coded video sequence(s) as createdby the entropy coder (645) to prepare for transmission via acommunication channel (660), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(640) may merge coded video data from the video coder (603) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (650) may manage operation of the video encoder (603).During coding, the controller (650) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of Intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A Predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A Bi-directionally Predictive Picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference pictures. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (603) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (603) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (640) may transmit additional datawith the encoded video. The source coder (630) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, Supplementary EnhancementInformation (SEI) messages, Visual Usability Information (VUI) parameterset fragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to Intra prediction) makes uses of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first and a second reference picture thatare both prior in decoding order to the current picture in the video(but may be in the past and future, respectively, in display order) areused. A block in the current picture can be coded by a first motionvector that points to a first reference block in the first referencepicture, and a second motion vector that points to a second referenceblock in the second reference picture. The block can be predicted by acombination of the first reference block and the second reference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels and the like.

FIG. 7 shows a diagram of a video encoder (703) according to anotherembodiment of the disclosure. The video encoder (703) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (703) is used in theplace of the video encoder (403) in the FIG. 4 example.

In an HEVC example, the video encoder (703) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (703) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (703) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(703) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (703) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 7 example, the video encoder (703) includes the interencoder (730), an intra encoder (722), a residue calculator (723), aswitch (726), a residue encoder (724), a general controller (721) and anentropy encoder (725) coupled together as shown in FIG. 7.

The inter encoder (730) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique.

The intra encoder (722) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform and, in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques).

The general controller (721) is configured to determine general controldata and control other components of the video encoder (703) based onthe general control data. In an example, the general controller (721)determines the mode of the block, and provides a control signal to theswitch (726) based on the mode. For example, when the mode is the intra,the general controller (721) controls the switch (726) to select theintra mode result for use by the residue calculator (723), and controlsthe entropy encoder (725) to select the intra prediction information andinclude the intra prediction information in the bitstream; and when themode is the inter mode, the general controller (721) controls the switch(726) to select the inter prediction result for use by the residuecalculator (723), and controls the entropy encoder (725) to select theinter prediction information and include the inter predictioninformation in the bitstream.

The residue calculator (723) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (722) or the inter encoder (730). Theresidue encoder (724) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (724) is configured to convert the residuedata in the frequency domain, and generate the transform coefficients.The transform coefficients are then subject to quantization processingto obtain quantized transform coefficients.

The entropy encoder (725) is configured to format the bitstream toinclude the encoded block. The entropy encoder (725) is configured toinclude various information according to a suitable standard, such asHEVC standard. In an example, the entropy encoder (725) is configured toinclude the general control data, the selected prediction information(e.g., intra prediction information or inter prediction information),the residue information, and other suitable information in thebitstream. Note that, according to the disclosed subject matter, whencoding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 8 shows a diagram of a video decoder (810) according to anotherembodiment of the disclosure. The video decoder (810) is configured toreceive a coded pictures that are part of a coded video sequence, anddecode the coded picture to generate a reconstructed picture. In anexample, the video decoder (810) is used in the place of the videodecoder (410) in the FIG. 4 example.

In the FIG. 8 example, the video decoder (810) includes an entropydecoder (871), an inter decoder (880), a residue decoder (873), areconstruction module (874), and an intra decoder (872) coupled togetheras shown in FIG. 8.

The entropy decoder (871) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example,intra, inter, b-predicted, the latter two in merge submode or anothersubmode), prediction information (such as, for example, intra predictioninformation or inter prediction information) that can identify certainsample or metadata that is used for prediction by the intra decoder(872) or the inter decoder (880) respectively residual information inthe form of, for example, quantized transform coefficients, and thelike. In an example, when the prediction mode is inter or bi-predictedmode, the inter prediction information is provided to the inter decoder(880); and when the prediction type is the intra prediction type, theintra prediction information is provided to the intra decoder (872). Theresidual information can be subject to inverse quantization and isprovided to the residue decoder (873).

The inter decoder (880) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (872) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (873) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (873) mayalso require certain control information (to include the QuantizerParameter QP), and that information may be provided by the entropydecoder (871) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (874) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (873) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (403), (603) and (703), and thevideo decoders (410), (510) and (810) can be implemented using anysuitable technique. In an embodiment, the video encoders (403), (603)and (703), and the video decoders (410), (510) and (810) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (403), (603) and (603), and the videodecoders (410), (510) and (810) can be implemented using one or moreprocessors that execute software instructions.

Described below are various embodiments of an intra prediction method(also referred to as intra prediction) according to the disclosedsubject matter.

In some video compression techniques, intra prediction is conducted on aplurality of samples, called henceforth a “unit” or a block, of a givenpicture. With respect to the plurality of samples in the unit, the unitcan be of any shape. The unit can be continuous or non-continuous. Insome video compression techniques, a shape of a unit is restricted to arectangular block of samples. Dimensions of the rectangular block can bepositive integers.

Some video compression techniques further restrict a size of therectangular block in each dimension, such as an X dimension and a Ydimension, to be a power of two (such as: four samples, eight samples,16 samples, . . . ).

Some video compression techniques further restrict a shape of therectangular block to be a square, that is, the size of the rectangularblock in the X and the Y dimension is equal. In some examples, the sizemay or may not be a power of two.

Before further describing the disclosure, the terms “available” and“availability” are introduced below. Consider a unit of samples. Thesamples can be arranged in a square, a rectangle, or any other suitableshape. Certain prediction samples or metadata of other predictionentities (such as prediction direction of surrounding units) may or maynot be “available” for prediction of samples or other predictionentities of the unit depending on, among other factors, a spatialposition of the unit in a picture, a coded bitstream structure includinga bitstream partitioned into slices, tiles, and so forth. In some videocoding technologies, a decoding order of units follows a scan order,i.e., left-to-right and top-to-bottom, thus potential referenceinformation and samples from units that follow the unit underreconstruction in the decoding order are naturally unavailable.Therefore, certain prediction data to the right or below the unit underreconstruction can be unavailable.

In some examples, certain prediction samples and other predictionentities may be unavailable even when the certain prediction samples areincluded in units that precede the unit under reconstruction in thedecoding order, and are located to the left or above the unit. Forexample, a sample or prediction entity may be unavailable when the unitunder reconstruction is at a boundary of a picture or a picture segment,for example, a slice or an (independent) tile. In various examples,slice and tile boundaries are treated, for the purpose of prediction, aspicture boundaries. Similarly, prediction entities other than predictionsamples may be unavailable even when the prediction samples areavailable, when the reference unit is coded in a mode that does notallow for generation or use of the prediction entity. For example, aprediction unit coded in a skip mode does not have a predictiondirection associated with the prediction unit.

When a prediction sample or prediction entity is not available, in atleast some cases, a value of the prediction sample or the predictionentity can be predicted from neighboring samples of the predictionsample. When the prediction samples/the prediction entities arepredicted, accuracy of the prediction method can be, for example,sub-optimal. However, as an encoder selects a prediction mode based onstate information that includes both prediction sample values and howthe prediction sample values are generated (including prediction fromsamples that are predicted due to unavailability), the encoder can use,at least in some cases, a mode that does not rely on intra prediction.In some examples, rate-distortion optimization techniques can be used toselect an appropriate mode.

Both predicted samples and available neighboring prediction samples can,for example, be luminance samples, chrominance samples, samples belongto a given color plane, and so forth. For example, in some videoprocessing systems employing video decoders, a video is sampled in aYCrCb 4:2:0 sampling structure with chrominance samples Cr and Cb beingsubsampled and handled in the respective chroma planes, thus, intraprediction can occur separately in each of the Y, Cr, and Cb colorplane. Control of the intra prediction can, in some cases, be generatedin an encoder based on the Y plane, signaled in a coded video bitstreamin relation to the Y plane, and applied in a decoder to a Y plane andCr/Cb plane separately. In another example, samples can be green samplesof a picture using a RGB color space, and red and blue samples arehandled in the respective R and B color planes. In other scenarios,intra prediction is performed for one or more color planes independentlyfrom other color planes, by both an encoder and a decoder. Othersuitable sampling structures, color spaces, and so forth, can also beused in intra prediction.

Referring to FIG. 9, in an embodiment, a unit under reconstruction(henceforth “block”) (901) may be of rectangular, but not square shape,thus, the block (901) under reconstruction can be referred to as anon-square block (901). In an example, the block (901) includes samplesS11-S14 and S21-S24 in a coded picture. In an example, samples in theblock (901) can be predicted based on intra prediction using predictionsamples in the same coded picture, such as prediction samples R01-R09and R10-R70, of the block (901). The intra prediction of the samples inthe block (901) can be performed along an intra prediction direction,also referred to as a prediction direction in the disclosure.

In some embodiments, such as shown in FIG. 9, a width of the non-squareblock (901) is from left to right, and a height of the non-square-block(901) is from top to bottom. Similarly, the first direction (915) pointsfrom left to right, and a second direction (925) points from top tobottom. The first direction (915) and the second direction (925) formfour quadrants, i.e., a bottom left quadrant I, a top left quadrant II,a top right quadrant III, and a bottom right quadrant IV. A diagonaldirection (928) points from top right to bottom left, and equallydivides the bottom left quadrant I and the top right quadrant III,respectively.

In general, intra prediction directions in video compression techniquescan be optimized for square blocks. Referring to FIG. 9, a second set(920) of intra prediction directions for the square blocks includesintra prediction directions (902-912). The second set (920) of intraprediction directions for the square blocks covers a second angularrange (924). In some embodiments, the second angular range (924) spansfrom 135° clockwise (CW) from an first direction (915) to 45°counterclockwise (CCW) from the first direction (915), thus covering anangular range of 180°. Referring to FIG. 9, the prediction direction902, referred to as a first end of the second angular range (924) is135° clockwise from the first direction (915). The prediction direction902 is parallel to the diagonal direction (928). The predictiondirection 912, referred to as a second end of the second angular range(924), is 45° counterclockwise from the first direction (915). Invarious embodiments, the second set (920) can include other predictiondirections not shown in FIG. 9. In general, the second set (920) ofintra prediction directions can include any suitable intra predictiondirections that, for example, are between the first end (902) and thesecond end (912). For example, there are 65 intra prediction directionsused in VVC.

Certain intra prediction directions that can be useful for square blockscan be less useful for non-square blocks, such as the block (901),because the spatial correlation between certain samples of the block(901) and prediction samples used based on the certain intra predictiondirections is low. For example, when the intra prediction direction(902) is used for intra prediction, samples S11 and S21 in the block(901), for example, can be predicted from respective reference samplesR20 and R30 that are direct neighbors of the block (901). Certainsamples of the block (901) are predicted from reference samples that arenot direct neighbors and can be relatively (spatially) far away from theblock (901). For example, the sample S24 is predicted from the referencesample R60, which is far away spatially. In various embodiments, intraprediction works well with a close spatial relationship betweenreference samples and samples to be predicted, for example, the samplesto be predicted are spatially close to the reference samples. Therefore,in various embodiments, an intra prediction direction such as the intraprediction direction (902) may not be chosen by a rate-distortionoptimized encoder for the non-square block (901).

In some embodiments, intra prediction directions are mapped to intraprediction modes (also referred to as prediction modes), and the intraprediction modes can be further mapped into codewords. As describedabove, certain intra prediction directions can be statistically morelikely to be used than other intra prediction directions. The certainintra prediction directions can be mapped into first codewords, theother intra prediction directions can be mapped into second codewords.Therefore, the first codewords can use less numbers of bits than thoseof the second codewords, thus, can be relatively shorter than the secondcodewords. In some examples, intra prediction directions/modes mapped tothe first codewords are referred to as “short” modes. The “short” modecan be a mode that is likely to be chosen by a rate-distortion optimizedencoder. For example, the intra prediction direction (902) cancorrespond to a short mode for the square blocks.

When using the same mapping of intra prediction directions to intraprediction modes as designed and optimized for the square blocks,valuable “short” modes for the intra prediction directions for thesquare blocks can be wasted for intra prediction directions, such as theintra prediction direction (902), for the non-square block (901). Asdescribed above, when there is a direct mapping between an intraprediction mode and a variable length codeword, a “short” mode can berepresented by a short variable length codeword.

In the same or another embodiment, certain intra prediction directionslocated at an end, such as the first end (902), of the second angularrange (924) of the second set (920) of intra prediction directions usedfor the square blocks, are unlikely to be chosen by a rate-distortionoptimized encoder for the non-square block (901) because of a lowspatial correlation between certain samples in the non-square block(901) and the corresponding reference samples used based on the certainintra prediction directions located at the end. Therefore, the certainintra prediction directions are not used in a first set of intraprediction directions for the non-square block (901). For example, forcertain block shapes, such as the non-square block (901) (see alsodescription below), the prediction directions between the first end(902) and the intra prediction direction (904) that is 22.5° CW from thefirst end (902) can form a subrange (922) of the second angular range(924), and the subrange (922) includes the intra prediction directionsthat are not used in the first set of intra prediction directions forthe non-square block (901). The subrange (922) can also be referred toas a removed subrange (922). In one example, the subrange (922) excludesthe intra prediction direction (904), and includes the first end (902)and any intra prediction directions in the second set (920), such as theintra prediction direction (903), that are between the first end (902)and the intra prediction direction (904). Referring to FIG. 9, the firstset of intra prediction directions for the non-square block (901) coversa first angular range (926) having two ends, the intra predictiondirections (904 and 912).

In the same or another embodiment, the subrange (922) that includes theintra prediction directions that are not used in the first set of intraprediction directions for the non-square block (901) can be dependent onspatial characteristics of the non-square block (901). In the same oranother embodiment, the spatial characteristics of the block (901) caninclude a shape of the block (901). In some examples, the shape of theblock (901) can be a relationship of sizes of the block (901) in the Xand the Y dimension, i.e., a first size in the X dimension, also thewidth from left to right, and a second size in the Y dimension, also theheight from top to bottom. As an example, in the same or anotherembodiment, for the block (901) with an aspect ratio of the first sizein the X dimension over the second size in the Y dimension being 2:1 asshown in FIG. 9, the subrange (922) can be between the intra predictiondirection (902) (135° CW from the first direction (915)) and the intraprediction direction (904) (22.5° CW from the intra prediction direction(902)) in the bottom left quadrant I.

When the aspect ratio is larger than 1, the subrange (922) can includethe first end (902) of the second set (920) and is located in the bottomleft quadrant I. When the aspect ratio that is larger than 1 increases,the subrange (922) can become larger, thus including more intraprediction directions in the second set (920).

In general, a subrange that includes intra prediction directions in thesecond set for the square blocks that are not used in a first set ofintra prediction directions for a non-square block can be determinedbased on symmetry along the diagonal direction (928).

In some embodiments, when an aspect ratio of a non-square block underreconstruction is less than 1, a subrange in the second set (920)includes the second end (912) of the second set (920) and is located inthe top right quadrant III. Note that the subrange in the second set(920) is not included in a first set of intra prediction directions forthe non-square block. In the same or another embodiment, for anon-square block with an aspect ratio of a first size in the X dimensionover a second size in the Y dimension being 1:2, a subrange can bebetween the second end (912) and the intra prediction direction (911) inthe top right quadrant III. In various embodiments, the subrangeincludes the second end (912). The second end (912) is opposite to thediagonal direction 928, and the intra prediction direction (911) is22.5° CCW from the second end (912). When the aspect ratio that is lessthan 1 decreases, the subrange can become larger, thus including moreintra prediction directions in the second set (920).

Referring to FIG. 10, shown is a non-square block or block (1001) of 1×4samples, representative of block sizes with an aspect ratio of 1:4.Similarly, the second set (920) of intra prediction directions for thesquare blocks includes intra prediction directions (902-912), as shownin the lower right of FIG. 10. In various examples, the second set (920)of intra prediction directions for the square blocks and the secondangular range (924) are identical to those shown in FIG. 9, thus,detailed descriptions are omitted for purposes of clarity. The fourquadrants I-IV are also identical to those in FIG. 9, thus, detaileddescriptions are omitted for purposes of clarity.

In various embodiments, a first set of intra prediction directions canbe used for the non-square block (1001), and the first set does notinclude intra prediction directions in a subrange (1022) of the secondangular range (924). As shown in FIG. 10, the subrange (1022) can bebetween the second end (912) and the intra prediction direction (910).The second end (912) is opposite to the diagonal direction 928 or 45°CCW from the first direction (915) and the intra prediction direction(910) is 11.25° CW from the intra prediction direction (909) that isopposite to the second direction (925). In various examples, thesubrange (1022) includes the second end (912), and excludes the intraprediction direction (910). Referring to FIG. 10, the first set covers afirst angular range (1026). In an example, the first angular range(1026) includes the first end (902) of the second set (920), the intraprediction direction (910), and other intra prediction directions of thesecond set (920) in between.

Other suitable subranges for other block shapes and/or aspect ratios ofnon-square blocks may include one or more intra prediction directions inthe second set (920) for the square blocks that are not used in a firstset for the non-square blocks.

In the same or another embodiment, intra prediction modes for the squareblocks corresponding to the intra prediction directions that are part ofthe removed subrange are not used for non-square blocks. As an example,in FIG. 9, an intra prediction mode associated with the intra predictiondirection (903) that is part of the subrange (922) is not used for thenon-square block (901). In some embodiments, a table of modes can beused to relate intra prediction directions and corresponding intraprediction modes used for a block. The table of modes for the non-squareblock (901) can be shortened accordingly, leading to a smaller numberused for the intra prediction modes and, thereby potential for optimizedentropy coding and better coding efficiency. In the same or anotherembodiment, the table of modes can be reordered according to alikelihood of the intra prediction directions still represented in thetable of modes.

In the same or another embodiment, when the intra prediction directionsin the removed subrange are not used for a non-square block, theassociated intra prediction modes can be re-assigned for other purposes,including, for example, signaling of previously unused directions, modesnot directly associated with a prediction direction, filter control offilters pertaining to reference samples or intra predicted samples, andso forth.

In the same or another embodiment, certain intra prediction directions,referred to as added intra prediction directions, are added to a firstset of intra prediction directions used for non-square blocks. A numberof the added intra prediction directions can be the same as a number ofintra prediction directions (referred to as removed intra predictiondirections) in the removed subrange described above. The added intraprediction directions can encompass a subrange referred to as an addedsubrange. The added subrange is included in a first angular rangecovered by the first set of intra prediction directions for thenon-square blocks. The added subrange can be of the same geometry as theremoved subrange, as described below.

FIG. 11 shows a non-square block (1101), a 4×2 block underreconstruction having an identical aspect ratio as that of thenon-square block (901) shown in FIG. 9. Further shown is the second set(920) of intra prediction directions for the square blocks. In variousexamples, the second set (920) of intra prediction directions for thesquare blocks and the second angular range (924) are identical to thoseshown in FIG. 9, thus, detailed descriptions are omitted for purposes ofclarity. Note that the intra prediction directions 902-912 for thesquare blocks are shown, however, for purposes of clarity, only theintra prediction directions 902-904, 911-912 are labeled in FIG. 11. Thefour quadrants I-IV are identical to those in FIG. 9, thus, detaileddescriptions are omitted for purposes of clarity.

Similarly, the intra prediction directions in the removed subrange orsubrange (922) are not included in a first set of intra predictiondirections for the non-square block (1101). The subrange (922) isidentical to that shown in FIG. 9, thus detailed description is omittedfor purposes of clarity. Two intra prediction directions (902, 903) inthe subrange (922) are depicted using dashed lines. The intra predictionmodes used to represent the intra prediction directions (902, 903) forthe square blocks can be re-used to represent, in this example, twoadded intra prediction directions (1106, 1107) in an added subrange(1108). Accordingly, the first set of intra prediction directions forthe non-square block (1101) covers a first angular range (1126) with afirst end (904) and a second end (1107). The first end or the intraprediction direction (904) is 22.5° CW from the diagonal direction(928), the second end (1107) is 22.5° CW from the intra predictiondirection (912) that is opposite to the diagonal direction (928). Thefirst angular range (1126) includes the added subrange (1108) and doesnot include the removed subrange (922). In some examples, such as shownin FIG. 11, the removed subrange (922) includes intra predictiondirections between the intra prediction directions (902) and (904).Further, the removed subrange (922) includes the intra predictiondirection (902) and does not include the intra prediction direction(904). On the other hand, the added subrange (1108) includes intraprediction directions between the second end (912) of the second set(920) and the added intra prediction direction (1107). Further, theadded subrange (1108) includes the added intra prediction direction(1107) and does not include the second end (912).

In some examples, such as shown in FIG. 11, the removed subrange (922)and the added subrange (1108) are symmetric with respect to the diagonaldirection (928). As described above, the removed subrange (922) coversan angular range of 22.5° CW from the diagonal direction (928), and theadded subrange (1108) covers an angular range of 22.5° CW from anopposite direction of the diagonal direction (928).

The added intra prediction directions can be beneficial when used forthe non-square block (1101) as shown in FIG. 11. For example, considerthe added intra prediction direction (1107). Using the intra predictiondirection (1107), a sample S11 in the non-square block (1101) can bepredicted from a reference sample R03, and a sample S14 can be predictedfrom a reference sample R06 that is adjacent to a direct neighbor R05 ofthe non-square block (1101). Therefore, the samples included in theblock (1101) can be predicted in the added intra prediction direction(1107), and the reference samples used to predict the samples in theblock (1101) are relatively close to the samples in the block (1101) ascompared to using an intra prediction direction in the removed subrange(922).

The benefit of the added subrange can come without an increase of anumber of intra prediction modes for the non-square block (1101) vis avis a number of intra prediction modes for the square blocks. In someembodiments, a number of intra prediction directions in the removedsubrange (922) is identical to a number of intra prediction directionsin the added subrange (1108), thus, a number of intra predictiondirections in the first set for the non-square block (1101) is identicalto a number of intra prediction directions in the second set (920) forthe square blocks. In some examples, as described above, intraprediction modes assigned to the intra prediction directions in theremoved subrange (922) for the square blocks are reassigned to the intraprediction directions in the added subrange (1108) for the non-squareblocks, thus, a number of intra prediction modes used for the first setfor the non-square block (1101) is equal to a number of intra predictionmodes used for the second set (920) for the square blocks.

In some examples, a number of intra prediction directions in the firstset for the non-square block (1101) can be less than a number of intraprediction directions in the second set (920) for the square blocks whena number of intra prediction directions in the added subrange is lessthan a number of intra prediction directions in the removed subrange. Insome other examples, such as shown in FIG. 9, a number of intraprediction directions in the first set can be less than a number ofintra prediction directions in the second set when the first set doesnot include the added subrange. Therefore, a number of intra predictionmodes for the non-square block (1101) is less than a number of intraprediction modes for the square blocks.

In some examples, in order to predict samples (S21-S24) in a second rowof the non-square block (1101) using the intra prediction direction(1106) or (1107), certain interpolation/filtering techniques canadvantageously be employed to predict the samples (S21-S24) from morethan one reference sample, or to avoid aliasing artifacts. Several suchinterpolation/filtering techniques include the one specified for certainintra prediction directions that are, for example, not multiples of 45°from the diagonal direction (928), such as specified in H.265. Forexample, the added intra prediction directions (1106 and 1107) are11.25° and 22.5° CW from an opposite direction of the diagonal direction(928) and, thus, are not multiples of 45° from the diagonal direction(928). On the other hand, the intra prediction direction 907 is 90° CWfrom the diagonal direction (928), thus, is a multiple (2 times) of 45°from the diagonal direction (928).

The symmetry considerations described above in the context of removingthe removed intra prediction directions can equally apply to adding theadded intra prediction directions. Similarly, a size of the addedsubrange can be dependent on the block shape of the block (1101) in asimilar way as a size of the removed subrange can be dependent on theblock size. As described above in reference to FIG. 9 when the aspectratio of the non-square block (901) that is larger than 1 increases, thesubrange (922) can become larger, thus including more intra predictiondirections of the second set (920).

FIG. 12 shows a flow chart outlining a process (1200) according to someembodiments of the disclosure. The process (1200) is used in intraprediction to generate a prediction block for a non-square block underreconstruction. In various embodiments, the process (1200) is executedby processing circuitry, such as the processing circuitry in theterminal devices (310, 320, 330 and 340), the processing circuitry thatperforms functions of the video encoder (403), the processing circuitrythat performs functions of the video decoder (410), the processingcircuitry that performs functions of the video decoder (510), theprocessing circuitry that performs functions of the intra predictionmodule (552), the processing circuitry that performs functions of thevideo encoder (603), the processing circuitry that performs functions ofthe predictor (635), the processing circuitry that performs functions ofthe decoder (633), the processing circuitry that performs functions ofthe intra encoder (722), the processing circuitry that performsfunctions of the intra decoder (872), and the like. The process startsat (S1201) and proceeds to (S1210).

At (S1210), a block size for a non-square block under reconstruction isobtained. For example, at least one syntax element from a coded videobitstream is decoded. The at least one syntax element is indicative ofthe block size of the non-square block. The block size includes a firstsize in a first dimension, such as a width of the non-square block, anda second size in a second dimension, such as a height of the non-squareblock, and the first size is different from the second size. In someexamples, additional information, such as a location of the non-squareblock within a picture under reconstruction, is obtained.

At (S1220), the processing circuitry determines an intra prediction modefor the non-square block. In various embodiments, the intra predictionmode corresponds to an intra prediction direction in a first set ofintra prediction directions for the non-square block. As described abovein reference to FIGS. 9-11, the first set of intra prediction directionsdoes not include one or more intra prediction directions of a second setof intra prediction directions for square blocks.

At (S1230), the processing circuitry selects a reference sample for asample in the non-square block based on the intra prediction mode.Referring to FIG. 11, in some examples, the non-square block is theblock (901), and the intra prediction mode represents the intraprediction direction (912). In order to predict, for example, the sampleS21 using the intra prediction direction (912), the processing circuitrydetermines the reference sample to be R03.

At (S1240), the processing circuitry determines whether the referencesample is available, as described above. When the reference sample isavailable, the process (1200) proceeds to (S1260). When the referencesample is not available, the process (1200) proceeds to (S1250).

At (S1250), the processing circuitry determines a value for thereference sample, for example, using a neighboring sample of thereference sample, as described above. For example, when the referencesample R03 is not available, other samples, such as R02, and the like,can be used to determine a value for the reference sample R03. Theprocess (1200) then proceeds to (S1260).

At (S1260), the processing circuitry predicts the sample in thenon-square block based on the reference sample. In some embodiments, avalue of the sample S21 is obtained based on the value of the referencesample R03. For example, the value of the sample S21 is equal to thevalue of the reference sample R03. In some examples, when the intraprediction mode represents, for example, an intra prediction direction(911), a value of the sample S21 can be determined based on more thanone reference sample, using interpolation, filtering, and the like. Thenthe process (1200) proceeds to (S1299) and terminates.

Note that the process (1200) can be suitably adapted. For example, at(S1230), the processing circuitry can select more than one referencesample for a sample in the non-square block based on the intraprediction mode when the intra prediction mode represents, for example,an intra prediction direction (910).

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 13 shows a computersystem (1300) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 13 for computer system (1300) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1300).

Computer system (1300) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1301), mouse (1302), trackpad (1303), touchscreen (1310), data-glove (not shown), joystick (1305), microphone(1306), scanner (1307), camera (1308).

Computer system (1300) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1310), data-glove (not shown), or joystick (1305), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1309), headphones(not depicted)), visual output devices (such as screens (1310) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (1300) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1320) with CD/DVD or the like media (1321), thumb-drive (1322),removable hard drive or solid state drive (1323), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1300) can also include an interface to one or morecommunication networks. Networks can for example be wireless, wireline,optical. Networks can further be local, wide-area, metropolitan,vehicular and industrial, real-time, delay-tolerant, and so on. Examplesof networks include local area networks such as Ethernet, wireless LANs,cellular networks to include GSM, 3G, 4G, 5G, LTE and the like, TVwireline or wireless wide area digital networks to include cable TV,satellite TV, and terrestrial broadcast TV, vehicular and industrial toinclude CANBus, and so forth. Certain networks commonly require externalnetwork interface adapters that attached to certain general purpose dataports or peripheral buses (1349) (such as, for example USB ports of thecomputer system (1300)); others are commonly integrated into the core ofthe computer system (1300) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (1300) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1340) of thecomputer system (1300).

The core (1340) can include one or more Central Processing Units (CPU)(1341), Graphics Processing Units (GPU) (1342), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1343), hardware accelerators for certain tasks (1344), and so forth.These devices, along with Read-only memory (ROM) (1345), Random-accessmemory (1346), internal mass storage such as internal non-useraccessible hard drives, SSDs, and the like (1347), may be connectedthrough a system bus (1348). In some computer systems, the system bus(1348) can be accessible in the form of one or more physical plugs toenable extensions by additional CPUs, GPU, and the like. The peripheraldevices can be attached either directly to the core's system bus (1348),or through a peripheral bus (1349). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (1341), GPUs (1342), FPGAs (1343), and accelerators (1344) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1345) or RAM (1346). Transitional data can be also be stored in RAM(1346), whereas permanent data can be stored for example, in theinternal mass storage (1347). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1341), GPU (1342), massstorage (1347), ROM (1345), RAM (1346), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1300), and specifically the core (1340) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1340) that are of non-transitorynature, such as core-internal mass storage (1347) or ROM (1345). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1340). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1340) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1346) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1344)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

APPENDIX A: ACRONYMS

MV: Motion Vector

HEVC: High Efficiency Video Coding

SEI: Supplementary Enhancement Information

VUI: Video Usability Information

GOPs: Groups of Pictures

TUs: Transform Units,

PUs: Prediction Units

CTUs: Coding Tree Units

CTBs: Coding Tree Blocks

PBs: Prediction Blocks

HRD: Hypothetical Reference Decoder

SNR: Signal Noise Ratio

CPUs: Central Processing Units

GPUs: Graphics Processing Units

CRT: Cathode Ray Tube

LCD: Liquid-Crystal Display

OLED: Organic Light-Emitting Diode

CD: Compact Disc

DVD: Digital Video Disc

ROM: Read-Only Memory

RAM: Random Access Memory

ASIC: Application-Specific Integrated Circuit

PLD: Programmable Logic Device

LAN: Local Area Network

GSM: Global System for Mobile communications

LTE: Long-Term Evolution

CANBus: Controller Area Network Bus

USB: Universal Serial Bus

PCI: Peripheral Component Interconnect

FPGA: Field Programmable Gate Areas

SSD: solid-state drive

IC: Integrated Circuit

CU: Coding Unit

JEM: Joint Exploration Model

VVC: Versatile Video Coding

BMS: Benchmark Set

CU: Coding Unit

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

What is claimed is:
 1. A method for video decoding in a decoder inaccordance with a video coding technology, the method comprising:decoding at least one syntax element from a coded video bitstream, theat least one syntax element indicative of a block size of a non-squareblock under reconstruction that has a rectangular shape, the block sizeincluding a first size in a first dimension and a second size in asecond dimension, the first size being different from the second size;determining a first set of intra prediction directions for thenon-square block based on the block size and from a second set of intraprediction directions for a square block, the first set of intraprediction directions including a first angular subrange of the secondset of intra prediction directions and not including a second angularsubrange of the second set of intra prediction directions, and each ofthe second set of intra prediction directions being included in one ofthe first angular subrange and the second angular subrange; andpredicting a sample of the non-square block based on the first set ofintra prediction directions for the non-square block.
 2. The method ofclaim 1, wherein the first set of intra prediction directions does notinclude one of the second set of intra prediction directions thatcorresponds to an end of an angular range covered by the second set ofintra prediction directions, the end of the angular range being an endof the second angular subrange.
 3. The method of claim 2, wherein intraprediction modes used for the first set of intra prediction directionsdo not include an intra prediction mode used for the one of the secondset of intra prediction directions.
 4. The method of claim 2, wherein anintra prediction mode used for the one of the second set of intraprediction directions is assigned to one of the first set of intraprediction directions, the one of the first set of intra predictiondirections not being included in the second set of intra predictiondirections.
 5. The method of claim 4, wherein in a combined angularrange covered by the first set of intra prediction directions and thesecond angular subrange, the one of the first set of intra predictiondirections corresponds to a first end of the combined angular range, andthe second angular subrange includes a second end of the combinedangular range, the second end being the end of the angular range coveredby the second set of intra prediction directions.
 6. The method of claim4, wherein another angular range is covered by the first set of intraprediction directions; and the one of the first set of intra predictiondirections is outside the angular range covered by the second set ofintra prediction directions.
 7. The method of claim 4, wherein a numberof the first set of intra prediction directions not included in thesecond set of intra prediction directions is equal to a number of thesecond set of intra prediction directions not included in the first setof intra prediction directions.
 8. The method of claim 1, wherein anumber of intra prediction modes used for the second set of intraprediction directions for the square block is equal to a number of intraprediction modes used for the first set of intra prediction directionsfor the non-square block.
 9. The method of claim 2, wherein the one ofthe second set of intra prediction directions is based on a shape of thenon-square block.
 10. The method of claim 2, wherein the one of thesecond set of intra prediction directions is based on an aspect ratio ofthe non-square block.
 11. An apparatus, comprising processing circuitryconfigured to: decode at least one syntax element from a coded videobitstream, the at least one syntax element indicative of a block size ofa non-square block under reconstruction that has a rectangular shape,the block size including a first size in a first dimension and a secondsize in a second dimension, the first size being different from thesecond size; determine a first set of intra prediction directions forthe non-square block based on the block size and from a second set ofintra prediction directions for a square block, the first set of intraprediction directions including a first angular subrange of the secondset of intra prediction directions and not including a second angularsubrange of the second set of intra prediction directions, and each ofthe second set of intra prediction directions being included in one ofthe first angular subrange and the second angular subrange; and predicta sample of the non-square block based on the first set of intraprediction directions for the non-square block.
 12. The apparatus ofclaim 11, wherein the first set of intra prediction directions does notinclude one of the second set of intra prediction directions thatcorresponds to an end of an angular range covered by the second set ofintra prediction directions, the end of the angular range being an endof the second angular subrange.
 13. The apparatus of claim 12, whereinintra prediction modes used for the first set of intra predictiondirections do not include an intra prediction mode used for the one ofthe second set of intra prediction directions.
 14. The apparatus ofclaim 12, wherein an intra prediction mode used for the one of thesecond set of intra prediction directions is assigned to one of thefirst set of intra prediction directions, the one of the first set ofintra prediction directions not being included in the second set ofintra prediction directions.
 15. The apparatus of claim 14, whereinanother angular range is covered by the first set of intra predictiondirections; and the one of the first set of intra prediction directionsis outside the angular range covered by the second set of intraprediction directions.
 16. The apparatus of claim 14, wherein a numberof the first set of intra prediction directions not included in thesecond set of intra prediction directions is equal to a number of thesecond set of intra prediction directions not included in the first setof intra prediction directions.
 17. The apparatus of claim 11, wherein anumber of intra prediction modes used for the second set of intraprediction directions for the square block is equal to a number of intraprediction modes used for the first set of intra prediction directionsfor the non-square block.
 18. A non-transitory computer-readable storagemedium storing a program executable by at least one processor toperform: decoding at least one syntax element from a coded videobitstream, the at least one syntax element indicative of a block size ofa non-square block under reconstruction that has a rectangular shape,the block size including a first size in a first dimension and a secondsize in a second dimension, the first size being different from thesecond size; determining a first set of intra prediction directions forthe non-square block based on the block size and from a second set ofintra prediction directions for a square block, the first set of intraprediction directions including a first angular subrange of the secondset of intra prediction directions and not including a second angularsubrange of the second set of intra prediction directions, and each ofthe second set of intra prediction directions being included in one ofthe first angular subrange and the second angular subrange; andpredicting a sample of the non-square block based on the first set ofintra prediction directions for the non-square block.
 19. The apparatusof claim 14, wherein in a combined angular range covered by the firstset of intra prediction directions and the second angular subrange, theone of the first set of intra prediction directions corresponds to afirst end of the combined angular range, and the second angular subrangeincludes a second end of the combined angular range, the second endbeing the end of the angular range covered by the second set of intraprediction directions.
 20. The apparatus of claim 12, wherein the one ofthe second set of intra prediction directions is based on a shape of thenon-square block.
 21. The apparatus of claim 11, wherein the intraprediction directions that are included in both the first set of intraprediction directions and the second set of intra prediction directionsare a plurality of adjacent intra prediction directions in the secondset of intra prediction directions.